Due to a trend of warmer winters in the state, Stewart’s Disease, also known as Stewart’s Wilt, should now be on the radar for Wisconsin sweet corn and seed corn producers. Warmer winters increase the overwintering survival rate of corn flea beetle, the primary vector of the bacterium that causes the disease.
Corn Flea Beetle and Stewart’s Disease: 2007 Areas at Risk in Wisconsin
Eileen Cullen, Extension Entomologist, Dept. of Entomology
Sarah Schramm, Associate Research Specialist, Dept. of Entomology
Due to a trend of warmer winters in the state, Stewart’s Disease, also known as Stewart’s Wilt, should now be on the radar for Wisconsin sweet corn and seed corn producers. Warmer winters increase the overwintering survival rate of corn flea beetle, the primary vector of the bacterium that causes the disease.
Corn flea beetles are small (1/16-inch long), shiny black beetles. They have enlarged hind legs enabling them to jump when disturbed. Corn flea beetles transfer Stewart’s Disease from one plant to another. The beetles pick up the bacterium by feeding on infected plant material. Once picked up, the bacterium lives inside the corn flea beetle’s gut for the rest of the beetle’s life, infecting any plant the beetle feeds upon. Since the beetles overwinter as adults, P. stewartii can be transmitted to corn crops from one year to the next.
Stewart’s Disease is primarily a concern in sweet corn varieties and dent corn inbred seed production fields as some varieties and inbred lines are highly susceptible to Stewart’s Disease. Therefore, planting resistant varieties is the most effective control measure. Many dent field corn hybrids have adequate partial resistance.
A higher survival rate of the vector increases risk of Stewart’s Disease being transmitted to the next year’s corn crop. The WI DATCP Pest Survey has reported a return of Stewarts Disease to Wisconsin after a 56-year absence. Although seed field inspections since 2000 have turned up only sporadic reports of Stewarts Disease, the reports do indicate that corn flea beetles and Stewart’s Disease are undergoing range expansion northward with milder winters (WI DATCP 2007).
Stewart’s Disease is caused by the bacterium Pantoea stewartii, formerly called Erwinia stewartii. The disease has two phases: a wilt phase and a leaf blight phase. The wilt phase generally occurs early in the season after corn flea beetles feed on young corn plants. Symptoms of disease transmission include linear water-soaked lesions, followed stunted plants with withered leaves (Esker and Nutter 2002). The leaf blight phase is more common after pollination but can occur at any point in the season (Pataky et al. 2005). In this phase, leaf lesions turn into brown, dead streaks in the leaves and leaf margins become wavy.
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Corn resistant (left) and susceptible (right) to Stewart’s Disease Photo Credit: J.K. Pataky, University of Illinois, Department of Crop Sciences.
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Corn flea beetle overwintering survival rate depends upon the severity of the winter. Two temperature-based indices are commonly used to estimate the level of risk of Stewart’s Disease. Both models integrate the average monthly temperatures for December, January and February to predict corn flea beetle overwintering survival and probable occurrence of Stewart’s Disease. The Iowa State Model assigns a risk level based on the sum of the mean monthly temperatures for December, January, and February (see Table 1).
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Sum of the Mean Monthly Temperatures
(Dec, Jan, Feb) in Fahrenheit
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Disease Risk
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90° or above
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High
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85° - 90°
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Moderate to High
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80° - 85°
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Moderate to Low
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Less than 80°
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Low
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Table 1. Iowa State Model for predicting Stewart’s Disease risk based on overwintering survival of the primary vector, the corn flea beetle.
The Stevens-Boewe Index assigns an anticipated severity level of the damage caused by corn flea beetle feeding. It uses the average monthly temperature of December, January, and February to estimate the severity of early season wilt and late-season blight (see Table 2).
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Average Monthly Temperature
(Dec, Jan, Feb) in Fahrenheit
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Severity of
Early Season wilt
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Severity of
Late Season Blight
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33° or above
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Severe
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Severe
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30° - 33°
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Moderate
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Moderate to Severe
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27° - 30°
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Light
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Light to Moderate
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Less than 27°
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Absent or nearly so
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A trace, at most
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Table 2. Stevens-Bowe Index for predicting severity of Stewart’s Disease based on overwintering survival of the primary vector, the corn flea beetle.
The WI DATCP Pest Survey recently reviewed the mean monthly temperatures (Dec. 2006, Jan. and Feb. 2007) for sites throughout Wisconsin and assessed both types of risk.
Using the more conservative Iowa State Model, the risk of Stewart’s Disease occurring is high in SE Wisconsin (Kenosha); moderate to high across Southern WI (Milwaukee, Racine, Afton, Brodhead, Watertown, Dodgeville, and East Central WI (Manitowoc); and low to moderate for Northwest (Gordon, Madeline Island), North Central (Hurley, Wausau, Rhinelander), Northeast (Goodman, Florence, Antigo), West Central (Eau Claire, La Crosse, Jim Falls), Central (Hancock, Stevens Point, Marshfield) and East Central WI near Appleton, and Green Bay.
Using the Stevens-Boewe Index, a moderate level of damage is predicted near Kenosha, and a low level of damage is predicted for all other locations.
Scouting sweet corn and inbred seed corn fields for presence of corn flea beetles from seedling emergence to V5 stage is a good practice. Check your variety and/or inbred planted to confirm host plant resistance status. Leaf feeding injury is usually not severe enough to cause economic damage. However, if corn flea beetle numbers are high and corn is not growing vigorously due to beetle feeding, treatment thresholds are available. In commercial hybrid corn prior to V5, 50 percent of plants with severe feeding injury and five or more beetles per plant. In seed corn on susceptible hybrids, treatment can be justified if 10 percent of the plants inspected have severe feeding injury (long, white/silvery streaks on the corn leaf surface) and two or more beetles per plant (Munkvold 2001).
References:
Cullen, E. 2005. Stewart’s Wilt outlook in WI corn for 2005—and ID tips on springtail versus corn flea beetle. Wisconsin Crop Manager 12(7): 52-53.
Esker, P.D., Nutter, F.W. 2003. Temporal dynamics of corn flea beetle populations infested with Pantoea stewartii, causal agent of stewart’s disease of corn. Phytopathology 93:210-218.
Pataky, J.K., Michener, P.M., Freeman, N.D. 2005. Rates of seed treatment insecticides to control Stewart’s Wilt on sweet corn. Plant Disease. 89(3): 262-268.